1. Field of the Invention
The present invention relates generally to power take-off units and, more particularly, to a power take-off unit employed with a transmission of a vehicle to provide four-wheel drive capability.
2. Description of the Related Art
Generally speaking, land vehicles require three basic components. These components comprise a power plant (such as an internal combustion engine), a power train, and wheels. The internal combustion engine produces force by the conversion of chemical energy in a liquid fuel into the mechanical energy of motion (kinetic energy). The function of the power train is to transmit this resultant force to the wheels to provide movement of the vehicle.
The power train's main component is typically referred to as the “transmission”. Engine torque and speed are converted in the transmission in accordance with the tractive-power demand of the vehicle. The vehicle's transmission is also capable of controlling the direction of rotation being applied to the wheels, so that the vehicle may be driven both forward and backward.
In larger or commercial vehicles, it is known to provide transmissions with power take-off units that provide a secondary source of torque, or rotational power, from the transmission to any of a variety of power driven accessories, implements, or devices. The power take-off unit and transmission can be structured to provide a selectable source of output power. The power take-off unit can also be structured to provide output power at a reduction or increase over that of the transmission output.
It is also known to provide vehicles with the ability to drive all four wheels (i.e., four-wheel drive) from the output of the transmission. Numerous approaches to providing four-wheel drive capability to a vehicle presently exist. Certain conventional approaches provide constant, or full-time, four-wheel drive that is not de-selectable. Other vehicle drive systems may provide part-time, user selectable four-wheel drive, or automatically selected part-time four-wheel drive. Still other four-wheel drive systems provide a combination of automatic and user selectable drive mechanisms.
One manner of providing four-wheel drive capability to motor vehicles has been to modify and adapt the traditional power take-off unit and power the forward wheels of a primarily rear wheel driven vehicle. One of the most notable approaches has been to employ a specific type of power take-off known as a “transfer case”. Transfer cases have been generally employed within a drive train having a longitudinally mounted engine and transmission that is designed to drive the rear wheels. The transfer case takes torque output from the transmission, and passes the torque to the rear wheels. The transfer case also provides a secondary torque output, which is directed forward to a front differential to distribute torque to the front wheels. In most cases, the transfer case is selectable to engage or disengage the front wheels from the drive train.
In recent years, the consumer demand for four-wheel drive capability has expanded into almost all types of and styles of motor vehicles. Satisfying this demand has been problematic in some types of vehicles, especially those with transverse mounted engines or transaxle type transmissions that drive the front wheels. In response, a number of other adaptations of power take-off units have evolved to provide drive capability to both the front and rear wheels.
The newer approaches to the design of power take-off units for four-wheel drive vehicles that are designed to work with a transverse mounted transmission most often first drive the front wheels and use the power take-off unit to selectively drive the rear wheels. Known power take-off units of this type have the disadvantage of being bulky and complex. This disadvantage of conventional four-wheel drive power take-off units arises from the inclusion of one or more clutch mechanisms and planetary gear assemblies that are employed to transfer torque to the drive assemblies that drive the front and rear wheels. These associated assemblies require a great number of operative parts to be incorporated into the four-wheel drive power take-off unit, making it a large supplemental component of the drive train. Additionally, providing the operative torque through one or more power take-off clutch devices and/or planetary gear assemblies increases the likelihood of mechanical failures.
Other examples of conventional four-wheel drive power take-off units move the clutching assemblies and planetary gear systems into the transmission and transaxle and, in some cases, completely integrate the power take-off and transmission together. In these examples, shifting the complexity and bulk of the power take-off unit into the transmission or transaxle or merely integrating the power take-off unit into the transmission or transaxle itself only further enlarges the size of the transmission and increases its complexity. The underlying disadvantages still remain in the vehicle's drive train.
In any case, the size and complexity of the conventional four-wheel drive power take-off units makes it quite difficult to mount these devices in smaller more compact vehicles where physical space and ground clearance is already at a premium. Additionally, a power take-off unit employed for providing four wheel-drive capability must be robust enough to transfer torque to both sets of drive wheels under periods of physically demanding driving conditions. In this manner, attempting to simply reduce the size of the conventional power take-off units to allow them to fit within the physical restraints of a smaller motor vehicle is unacceptable as their load carrying capability is similarly reduced.
Therefore, it is desirable to provide a power take-off unit to be employed with a transverse transmission or transaxle of a motor vehicle that has reduced size and complexity compared to conventional units by reducing the number of parts, the overall size, and the cost. It is also desirable to provide a power take-off unit that employs a direct drive to one set of drive wheels without clutching mechanisms or planetary gear assemblies. It is further desirable to provide a power take-off unit that provides torque to all four wheels and is a separate unit from the transmission or transaxle. Therefore, there is a need in the art to provide a power take-off unit for use with a transverse mounted transmission or transaxle to provide four-wheel drive capability that meets these desires.